Printed capacitive sensor

ABSTRACT

The invention relates to a flexible, resilient capacitive sensor suitable for large-scale manufacturing. The sensor includes a dielectric, an electrically conductive detector and trace layer on the first side of the dielectric layer including a detector and trace, an electrically conductive reference layer on a second side of the dielectric layer, and a capacitance meter electrically connected to the trace and to the conductive reference layer to detect changes in capacitance upon interaction with detector. The sensor is shielded to reduce the effects of outside interference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/352,107filed on Feb. 10, 2006, now U.S. Pat. No. 7,208,960, the content ofwhich are incorporated by reference herein in their entirety.

TECHNICAL FIELD

This invention relates to a flexible capacitive sensor. Moreparticularly, the invention relates to a capacitive sensor suitable forlarge-scale manufacturing, that is both physically flexible and flexiblein its applications, and that senses incremental changes in pressurebased on the changes in the capacitance of the sensor.

BACKGROUND

Sensors, as the term is used here, refer to systems that react to achange in the environment. Pressure sensors react to an applied force orpressure using a variety of physical principles. Optical sensors changetheir optical properties under applied force. Similarly, electricallyresistive, or simply resistive, sensors have an electrical resistancethat changes under applied force. Piezoresistive sensors measure thechange in electrical resistance of a piezoresistive material as pressureis applied.

Capacitive sensors change capacitance. This can be in response to anapplied force; it can also be in response to the proximity of an objectwith relatively large capacitance, such as a person. Capacitive sensorscan also use a combination of resistive and capacitive sensing, in whichthe electrical resistance is measured when the capacitance changes.

Capacitive sensors are known and are used, for example, in touch screensand elevator buttons. The change in capacitance is typically based onone of two principles. The first approach involves changing thecapacitance monitored by the sensing system through direct electricalcontact with a large capacitive object, usually a person through theirfinger. In certain cases this type of sensor may also function to detectthe proximity of an object to the touch sensor, not requiring physicalcontact with the touch sensor. These systems often require directcontact between the person and the sensing system and may not work, iffor example the person is wearing a glove. Additionally, capacitivecoupling may not be well suited to quantitatively measuring the appliedpressure or proximity, but are capable of binary (on/off) sensing.

Another approach uses two conductive planes separated by a compressible,resilient dielectric. This composite forms a capacitor whose capacitancedepends in part on the distance between the conductive planes. Thecompression of the dielectric under pressure changes the capacitancebetween the planes, which can be detected by the sensing system. Bycalibrating the compression with the applied force or pressure, thissystem can be used to quantify the force or pressure of the interactionwith the sensor.

In recent years, there has growing interest in so-called “smart fabrics”that give electronic devices physical flexibility. They allow anelectronic device to be incorporated into an existing fabric rather thanhave a separate electronic device. An example of a smart fabric is acomputer keyboard that can be rolled up when not in use.

Flexible sensors are needed for smart fabrics and other applicationsthat require flexibility. Flexible optical pressure sensors have beendescribed, for example, in U.S. Pat. Nos. 4,703,757 and 5,917,180.Flexible sensors based on electrical contact of two or more conductingplanes are available from Eleksen Ltd. of Iver Heath, United Kingdom.Flexible pressure sensors that use principles of piezoresistance areavailable from, Softswitch Ltd. of likely, United Kingdom. A flexiblecapacitive sensor based on the capacitance of the human body isdescribed in U.S. Pat. No. 6,210,771. A flexible capacitive sensor thatuses the change in spacing between conductive planes is described in aseries of U.S. patents to Goldman, et al, including U.S. Pat. No.5,449,002. These patents teach the use of flexible conductive anddielectric layers, but they do not teach a system which can be used todetermine location, nor do they teach systems with multiple sensors(beyond the simple case of replications of a single sensor).

Thus there remains a need for a large-area flexible capacitive pressuresensor with good spatial resolution, capable of quantifying appliedpressure or force. Here we address those issues by describing multiplemethods of constructing a flexible capacitive sensing system withmultiple sensors that detects the presence of an applied force orpressure and is capable of determining the magnitude and location of theapplied force or pressure. All patent documents referenced in thisspecification are hereby specifically incorporated by reference in theirentirety as if fully set forth herein.

SUMMARY OF THE INVENTION

The present invention overcomes many of the deficiencies of capacitivetouch sensors. The present invention provides an inexpensive, lightweight, flexible, capacitive sensor and an efficient, low cost method ofmanufacturing.

According to its major aspects and briefly recited, the presentinvention is a capacitive sensor suitable for large-scale manufacturing,that is both physically flexible and flexible in its applications, andthat senses incremental pressure based on the changes in the capacitanceof the sensor.

An important advantage of the present invention is the way thecomponents, namely, the detector-and-trace layer, dielectric layer,conductive reference layer and penetration connector can be assembled toform the present capacitive sensor in a large-scale manufacturingprocess. Coating, gluing, and screen printing operations can be easilyautomated. Such operations can make a very large capacitive sensor arrayor a large fabric from which individual sensors or sensor arrays can becut.

Another important feature of the present invention is compatibility withthe use of penetration connectors for quickly and easily connectingtraces and the reference layer to a capacitance meter (an electricalmeasurement system) so that electrical signals can be applied ormeasured from the present sensor without the need for customizedelectrical connections.

Still another feature of the present invention is the use of capacitancerather than resistance for sensing contact. Resistance typicallyrequires the two conductive surfaces to touch; capacitance, in someembodiments not only does not require touching but does not even requirephysical contact with the sensor in some embodiments of the presentinvention, but mere proximity of a button with the user's finger.Capacitance may also be used to measure the pressure of contact and notjust the fact of contact.

These and other features and their advantages will be apparent to thoseskilled in the art of electrical circuits and capacitive circuits from acareful reading of the Detailed Description of Preferred Embodimentsaccompanied by the following drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings which are incorporated in and which constitutea part of this specification illustrate several exemplary constructionsand procedures in accordance with the present invention and, togetherwith the general description of the invention given above and thedetailed description set forth below, serve to explain the principles ofthe invention wherein:

FIG. 1 is an illustrational schematic view of a capacitive sensor with aelectrically conductive reference layer, a flexible, resilientdielectric layer and a detector and trace layer, all connected to acapacitance meter.

FIG. 2 is an illustrational schematic view of a capacitive sensor withmore than one trace and more than one detector.

FIG. 3 is an illustrational schematic view of a capacitive sensor withan additional dielectric layer and conductive reference layer.

FIG. 4 is an illustrational schematic view of a capacitive sensor withholes in the electrically conductive reference layer that overlap thedetectors in the detector and trace layer.

FIG. 5 is a schematic cross-section of a capacitive sensor withadditional dielectric and conductive reference layers as shown in FIG.3, additionally with optional outer layers.

FIG. 6 is an illustrative conductive pattern that could be used on theconductive layer, showing detectors, traces, and reference layerconnections.

To the extent possible, like elements are designated by like referencenumbers throughout the various views.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows one embodiment of the capacitive pressure sensor of theinvention. The flexible capacitive sensor 2 has a dielectric layer 6with a conductive reference layer 8 on one side and a detector and tracelayer 4 on the other side of the dielectric layer 6. The conductivereference layer 8 and the trace(s) 12 of the detector and trace layer 4are connected to a capacitance meter 14.

The flexible capacitive sensor 2 experiences a change in capacitanceupon the application of force sufficient to compress the sensor. Theamount of applied force, up to a point, is related to the extent of thechange in capacitance. In an alternate embodiment, the resistance isalso measured to determine the location of user interaction on thesensor. A capacitance meter 14 monitors the present flexible capacitivesensor to determine whether there has been a change in capacitance andthe extent of that change.

The dielectric layer 6 is a flexible, resilient layer or film.“Flexible”, as related to this invention, is defined to mean pliable andcapable of being substantially bent through its thinnest dimension andreturning to a flat configuration. Preferably, each layer in the sensoris flexible. “Resilient” is defined to mean a material that essentiallyreturns to its initial thickness after each of multiple compressions,either over a portion of the material or its entirety. Dielectric, inthis application, means a material that does not allow current to flowand supports an electric field even under the presence of a potentialdifference. A “film” or “foam” is defined to be a flexible material thatis essentially two dimensional in extent, that is, having a thickness inone dimension that is significantly smaller than its length or width.Foams include void spaces in a significant portion of their interior andare thus usually highly compressible. Films are defined to have few orno void spaces.

The resilience of the dielectric layer 6 is important for repeated useand durability and the flexibility is important so that the sensor maybe used in applications that require flexibility, such as fitting arounda molded dashboard, or on clothing as part of a smart fabric.Preferably, dielectric layer 6 is capable of bending to a radius ofcurvature ranging from 20 millimeters (mm) to 5 mm, preferably to arange of 10 mm to 4 mm, and more preferably to a range of 5 mm to 1 mm.

In one embodiment of the invention, the dielectric layer is a thin,flexible, resilient film that has a thickness of less than 250micrometers, preferably between 8 and 250 micrometers, and for someapplications, between 8 and 50 micrometers. This thin film isessentially free of voids (which can be filled with air or another gas),meaning that the film does not contain foam. The thin film may be asilicone film, such as 7 mil (approximately 175 micrometers) thickDuraflex PT9300 film available from Deerfield Urethane of SouthDeerfield, Mass. Compressibility enables the capacitance of sensor to bealtered by an applied force. The dielectric thin film preferablycompresses by 50% when a load of between 50 and 150 bars is applied.This range enables an acceptable signal to be read by the capacitancemeter.

In another embodiment, the dielectric layer 6 may be a flexible,resilient, and highly compressible closed or open cell foam. Some foamedmaterials include, but are not limited to, polyurethane foams, silicone,and rubber. The dielectric foam preferably compresses by 50% when a loadof between 0.5 and 1.0 bars is applied.

In another embodiment of the invention, the dielectric layer is aflexible, resilient spacer fabric. “Spacer fabric” as defined in thisapplication is a fabric that has upper and lower ground layers separatedby a gap that is supported by spacing yarns or fibers. The spacer fabricor other layers of fabric in the construction can be a woven, knitted,non-woven material, tufted materials, or the like. In some embodiments,the spacer fabrics is a double-needlebar knit, needled nonwoven fabric,or a hi-loft nonwoven fabric in which some of the fibers arepurposefully oriented in the vertical direction. The textile may beflat, or may exhibit a pile. In some embodiments, the spacer fabric canhave a thickness of between 1 mm and 10 cm, preferably between 1 mm and1 cm. Such textile materials can be formed of natural or syntheticfibers, such as polyester, nylon, wool, cotton, silk, polypropylene,rayon, lyocell, poly(lactide), acrylic, and the like, including textilematerials containing mixtures and combinations of such natural andsynthetic fibers. The spacer fabric preferably compresses by 50% when aload of between 0.07 and 1.4 bar is applied and compresses between 10and 50% when a 0.14 bar load is applied. These ranges enable anacceptable signal to be read by the capacitance meter.

The electrical resistance across the dielectric layer 6 (from one sideof dielectric layer 6 to its opposing side) is preferably 10⁹ ohms orgreater. The greater the dielectric constant of the dielectric layer,the greater the capacitance of capacitive pressure sensor 2. This mayallow the sensor to discriminate smaller signals, hence smaller appliedforces, making the system more sensitive.

The detector and trace layer 4 has one or more detectors 10 and traces12 and is flexible. Detectors 10 are local areas of conductive materialand traces 12 are continuous lines (that may be straight or curved) ofconductive material running from detectors 10 to an edge 16 of thedetector and trace layer 4. Each detector 10 is preferably electricallyconnected to a separate trace 12 and electrically isolated from otherdetectors and traces. Detectors 10 may also be referred to as buttons.

In some embodiments, there is more than one detector 10 and more thanone trace 12. Preferably, each detector has its own trace and thedetector and trace are electrically isolated from other detectors andtraces, shown for example in FIG. 2. In FIG. 2, the detector and tracelayer 4 is separated from the dielectric layer 6 such that the detectorand trace configuration may be seen. Detectors 10, 32, and 36 areconnected to traces 12, 34, and 38 respectively. Connections to thecapacitance meter can be made through a penetration connector (notshown) with separate pins for each trace, and other than through thecapacitance meter none of the detector/trace pairs are electricallyconnected to any other detector/trace pair.

Preferably, the conductivity measured from the center of detector 10 tothe point where trace 12 reaches edge 16 of detector and trace layer 4is one megaohm or less, and more preferably between 0 and 10,000 ohms.However, it is sufficient that the electrical resistance of detector 10to the end of trace 12 be less than the electrical resistance acrossdielectric layer 6.

The detector and trace layer 4 may be formed by applying conductivecoatings to the dielectric layer 6 or a separate layer. The separatelayer may be a fabric or film that is then applied to dielectric layer 6by laminating in any manner known to those skilled in the art.Preferably, an adhesive is used between the layers including reactiveurethane adhesives or low-melt polymeric materials. Adhesives can beapplied for example by roto-gravure printing, knife coating, powderapplication, or as a web, depending on the form of the adhesive.

In one embodiment of the invention, detectors 10 and traces 12 arescreen printed directly onto the dielectric layer 6 or on a film orfabric adhered to dielectric layer 6. The ink may be any conductive inkwhich is typically formed by blending resins or adhesives with powderedconductive materials such as, gold, silver, copper, graphite powder,carbon black, nickel or other metals or alloys. They may also becarbon-based ink, silver-based ink, or a combination of carbon-based andsilver-based inks. The conductive ink may be coated on the substrateusing any of a variety of methods known in the art, including but notlimited to, screen printing, applying by brush, applying by roller,spraying, dipping, masking, vacuum plating, vacuum deposition or anycombination of the foregoing.

The electrically conductive reference layer 8 of the flexible capacitivesensor 2 may be a conductive coating on the dielectric layer 6, aninherently conductive film or fabric, or an electrically conductivecoating on a film or fabric which is then adhered to the dielectriclayer 6. In some configurations the electrically conductive referencelayer is preferably continuous. In others, it may have openings in thelayer if desired. Preferably, the reference layer is flexible.

In one embodiment, the electrically conductive reference layer 8 is anelectrically conductive coating onto the dielectric layer. This enablesthe sensor to be thinner and weigh less, important for portableapplications and may also simplify assembly or reduce cost. Thematerials disclosed for the detector 10 and trace 12 may be also usedfor the conductive reference layer 8. In another embodiment, theelectrically conductive reference layer 8 is an inherently conductivefilm or fabric. Some inherently conductive films and fabrics include,for example, metallized fabrics, carbon-loaded olefin films, fabricscoated with conductive polymers, fabrics constructed from flexible,conductive yarns such as stainless steel yarns, and silver-coated yarns.In another embodiment, the electrically conductive reference layer 8 maybe a film or fabric with an electrically conductive coating. Preferably,the film or fabric is adhered to dielectric layer 6 preferably using athermoplastic, thermoset, pressure sensitive or UV curable adhesive.

The electrical resistance of the conductive reference layer 8 istypically less than less than 10,000 ohms. If the capacitive sensor 2 isnot being used to determine the position of the user interaction, thenthe electrical resistance of the conductive reference layer may be aslow as practical.

Additionally, the electrically conductive reference layer 8 can bepatterned with additional detectors and traces (not shown) placed inregistration with detectors 10 and traces 12 of the detector and tracelayer 4, rather than be a continuous layer. However, this approach doesintroduce an additional manufacturing complication to position theconductive reference layer 8 such that its detectors 10 and traces 12are registered with the pattern of detectors and traces in the detectorand trace layer 4.

In one embodiment, shown in FIG. 4, the electrically conductivereference layer 194 has a hole 192 in the layer that at least partiallyoverlaps the detector 110 in the detector and trace layer 104.Preferably, the hole 192 completely overlaps and is in alignment withthe detector 110. There may be 2 or more detectors and 2 or more holes,with each hole overlapping a detector.

In this configuration, the electric field lines of the capacitive sensoroccupy the space above the detector 110 and below the hole 192. They areeasily perturbed by an external capacitive object, such as a person'sfinger, that approaches or enters the hole 192. This perturbation willchange the capacitance sensed by the capacitive sensor and can bedetected as an event. The perturbation is caused even though thecapacitive object does not come into electrical contact with thedetector 110. The conductive reference layer 194 of the presentinvention shields the detector and trace layer 104 from the externalcapacitive object. A hole in the conductive reference layer 194overlapping the detector 110 concentrates the field lines in the areaabove the detector making the system more sensitive to events occurringat the detector 110 and less prone to false indications due toperipheral approach to the detector. Additionally, a capacitive sensorthat does not include a conductive reference layer will be moresusceptible to interference from external electromagnetic fields, straycapacitance, static electricity, and to false events due to contact ofthe external capacitive object with the trace.

The dielectric layer 106 in the capacitive sensor 190 shown in FIG. 4does not need to be compressive or resilient, since a change incapacitance can be caused by the proximity of a capacitive body, asopposed to a change in the distance between detector 110 and referencelayer 194. The dielectric layer 6 can be any suitably thin, flexible,electrically resistive material.

FIG. 3 shows a capacitive sensor 62 with an additional flexibledielectric layer 64 and conductive reference layer 66. The secondflexible, resilient dielectric layer 64 is on the detector and tracelayer 4 on the side opposite the original dielectric layer 6. There is asecond electrically conductive reference layer 66 adjacent to the seconddielectric layer 64 on the side opposite the detector and trace layer 4.The materials used for the second flexible, resilient, dielectric layerand the second electrically conductive reference layer may be the samematerials and have the same physical properties as the dielectric layerand conductive reference layer described previously.

Other layers may be applied to the sensor such as an insulating layerand are preferably flexible. An insulating layer may be coated,laminated, stitched, or otherwise applied to either or both of exteriorsurfaces of the capacitive sensor 2, 30, 62, or 190. These layers may beconstructed of any materials and in any manner such that the overallflexibility of the sensor remains acceptable. Usually these materialswill retain the thin profile that is typical of the capacitive sensorsof the invention. Possible materials for the exterior layers includetextiles, leather or other hides, films, or coatings. The insulatinglayers may each be a composite of multiple materials and layers, and thetop and bottom insulating layers need not be of the same make-up.

Decorative graphics or information, e.g., information about, orinstructions for, touch sensor or the display or other device to whichtouch sensor is applied or connected, may be printed on an outermostinsulating layer on the sensor. Typically the top surface of the sensor,the surface presented to the user, will include graphics to indicate thelocation and function of each of the detectors. The material can bechosen to provide both decorative and functional aspects. Functions ofthe insulating layer may include visual or tactile aesthetics,resistance to abrasion or punctures, stain repellence, protection fromspills and liquids, resistance to ultraviolet degradation, etc. Thebottom layer of the sensor can be made with similar materials to servefunctions similar to the top layer, except that decorative orinformative graphics are typically not included.

For the capacitive sensor 62, the capacitance meter 14 is connected tothe conductive reference layer 8, the second conductive reference layer66 and each trace 12. The conductive reference layer 8 is at a firstvoltage, the trace 12 is at a second voltage, and the second referencelayer 66 is at a third voltage, where the first and second voltages havea difference of at least 0.1 volts and the second and third voltageshave a difference of at least 0.1 volts. In one embodiment, the firstand second voltages have a difference of at least 1.0 volts and thesecond and third voltages have a difference of at least 1.0 volts.Preferably, the first and third voltages are equal. In one embodiment,the first and third voltages form the reference voltage and are heldconstant during the operation of the capacitive sensor. In oneembodiment, the reference voltage is held equal to earth ground or theground of the sensor environment. This will serve to best isolate thecapacitive sensor from external interference and electrical discharges.

The first conductive reference layer 8 and the second conductivereference layer 66 each combine with the detector and trace layer 4 toform two separate capacitors. Preferably, the first and third voltageson each of the conductive reference layers are equal so that the twoseparate capacitors are electrically parallel. This simplifies therequirements of the meter 14, which can treat the two separatecapacitors as a single capacitor of larger capacitance. Largercapacitance will also typically improve the sensitivity of the sensor,which is one advantage of including conductive reference layers on bothsides of the detector and trace layer 4. The second electricallyconductive reference layer 66 also helps shield the sensor frominterference in the same manner as the first electrically conductivereference layer 8.

If the first electrically conductive reference layer 8 has a hole overeach of the detectors in the detector and trace layer 4 and thedielectric 6 is not compressible and resilient, then the secondelectrically conductive reference layer 66 will act primarily to helpshield the sensor from interference.

In the case of a sensor built with a compressible dielectric,capacitance of the sensor varies inversely with the compression ofdielectric layer 6. A force applied to detector 10 will compressdielectric layer 6 thus increasing the capacitance between detector andtrace layer 4 and electrically conductive reference layer 8. When theforce is removed, or merely lessened, the separation distance betweendetector and trace layer 4 and conductive reference layer 8 increasesand the capacitance of capacitive sensor 2 decreases.

In the case where the dielectric is not compressible but there is a holein the conductive reference layer 4 that overlaps the detector 10 in thedetector and trace layer 4, the capacitance increases with the approachof a capacitive body such as a person's finger. In both cases, thechange in capacitance can be monitored by the meter 14, which cansubsequently initiate a desired response, such as activation of anelectrical device such as a radio.

To monitor the change in capacitance, preferably, a first voltage isapplied to the conductive reference layer 8 and a second voltage isapplied to the trace 12. In the case that there is more than one traceon the detector and trace layer 4, then each trace would get a separatevoltage (ex. second, third, forth, fifth, etc. voltage). In the casewhere there is more than one trace, preferably the voltages are appliedto the traces sequentially. In one embodiment, the voltages are appliedsequentially and are substantially equal. Preferably, the voltagesapplied to the conductive reference layer are at least 0.1 voltsdifferent than the voltages applied to the trace(s), or in anotherembodiment, more than 1 volt different.

At the edge of the detector and trace layer 4, a penetration connector(not shown) is used to make electrical contact with traces 12. Theprinciple of operation of penetration connectors is well known inelectronics. When making electrical connection with electricalconductors coated with insulation, penetration connectors are used to“bite” through the insulation to the conductor inside. Penetrationconnector will have teeth, which are applied to the trace 12 and to theconductive reference layer 8, and potentially to the conductivereference layer 66 if one exists. In one embodiment, the traces extendpast the other layers to be connected to more easily. In anotherembodiment of the present invention having a plurality of detectors 10and traces 12, separate teeth in the connector can contact each of theseparate traces so that the meter 14 can be used to sense changes incapacitances as pressure is applied to each detector or multipledetectors. The use of penetration connector simplifies manufacture on alarge scale.

The penetration connector allows connection of the present flexiblecapacitive sensor 2 to the capacitance meter 14 by connecting the meter14 to the trace 12 and the meter 14 to the conductive reference layer 8.The capacitance meter 14 measures the voltage across dielectric layer 6from detector 10 to the conductive reference layer 8 and compares thatvoltage to a reference voltage. If the capacitance across the dielectriclayer 6 at detector 10 changes, the voltage across detector 10 alsochanges, and a voltage output signal is generated based on thedifference between the reference voltage and the nominal voltage acrossdetector 10. As the force applied to detector 10 is reduced, anddielectric layer 6 expands to its original dimensions, capacitancedecreases.

The capacitance of detectors in this arrangement can be measured by avariety of electrical methods, two of which will be discussed here. Theelectrical measurements make use of the fact that the resistance of thetraces does not change, only the capacitance of individual detectors.Thus, the measurable RC time constant characteristic of each detectorand trace combination changes only due to changes in capacitance of thedetector. One method is a voltage shift method; the other is a phaseshift in the frequency response.

In the first method, which we will refer to as the voltage shift method,we use a series resistor connected to the trace. The capacitance meter14 looks for any one of the following: (1) the time to obtain a setdecline in the voltage of the trace and detector during discharge ofdetector 10; (2) the decline in the voltage of the trace and detectorduring a set time from the beginning of the discharge of detector 10;(3) the time to obtain a set increase in the voltage of the trace anddetector during the charging of detector 10; or (4) the increase in thevoltage of the trace and detector during a set time from the beginningof the charge of detector 10. Any one of these four quantities allowsdetermination of the RC time constant, and hence a measurement of thechange in capacitance of the detector.

In the phase shift method, a time-varying voltage signal is applied tothe detector and trace layer 4. A resistor to ground is connected to theconductive reference layer 8. The resistor is used to measure the phaseshift between the applied signal and the lagging signal through thedetector and trace layer 4. As the lag is caused by the presence ofcapacitance in the detector and trace layer 4, a change in the lag canbe used to determine the change in capacitance. The amplitudes of theoriginal and lagging signal can be compared to yield more informationabout the state of the system. As is known in the art, common forms ofthe voltage signal include impulses, sine waves, and square waves.Preferably alternating voltage signals will have a frequency greaterthan 10 kHz.

The digital information, the decay time constant or the phase shift,represents the continuous time variation of the resistive-capacitiveproperties of the network and, as such, the conditions of detector 10.To achieve a better signal-to-noise ratio, averaging and filtering maybe applied to the continuous data stream.

The time constant method and the phase shifts are prone toelectromagnetic interference as well as stray capacitance. Thus, thenoise content of the signals can obscure true conditions. Sampling isperformed intervals defined by settable interrupts in microcontroller.Through sampling dictated by the Nyquist criterion, which governssampling theory and digital reconstruction of high-frequency events,events happening at less than half the sampling frequency can besuccessfully captured. At the time of individual sampling, multiplesamples on the order of a few microseconds each are averaged together toreduce the error introduced by the analog-to-digital converter as wellas small electromagnetic effects. Sampling may occur at regular timeintervals, or it may be advantageous to sample at random intervals sothat the noise spectrum is not well correlated with the samplinginterval.

The sampled data are then passed into either finite impulse responsefilters or infinite impulse response filters. These filters furtherreduce the effects of noise and interference on the sampled data fromsources such as power lines. In this manner, a better estimate of thecapacitance of the detector can be determined through a better estimateof the phase shift or time constant.

Cascading different filters permits different interpretations of thedata. For example, a set of filters is used to remove or ignore longterm changes to the system (e.g., gradual loss of resilience in thedielectric layer 6), thus providing a stable baseline, while otherfilters isolate the short term changes (i.e., pressing detector 10). Theselection of different filters is a significant improvement over simplesampling and comparison to a threshold.

The capacitive sensor 14 requires calibration. Calibration is neededbecause baseline capacitance tends to drift over time because ofenvironmental changes, material changes, and external electromagneticfields. Particularly in dielectric materials made of foam,notwithstanding the use of foams with minimized creep and hysteresis,capacitance nonetheless will change in time. A sensor that can berecalibrated will always be more robust and sensitive than one thatcannot be.

There are three ways to calibrate sensor 14. The first way is to programcalibration settings at the time of manufacture. A second method is tocalibrate sensor 14 every time the system of which it is a partinitializes itself, that is, upon start up. This method effectivelyreduces errors for some variations on large time scales. In the thirdmethod, the sensor 14 is continuously calibrated for changing conditionsby filtering out extraneous electrical noise as well as disregarding theinadvertent touch or other contact. There are commercially availableelectronic modules that are designed to sense capacitance and thatincorporate continuous self-calibration, noise filtering andrecalibration.

EXAMPLES Example 1

A switch panel was made by laminating together multiple layers as shownin FIG. 5. All percentages are by weight unless otherwise specified.

Two identical resilient structures were made, consisting, in order, of:

-   -   I) a protective layer 104 of 100 g/m² CelFil 100 spunbonded        polyester nonwoven fabric from Polymeross y Derivados of Mexico,    -   II) a 1^(st) conductive layer 101 of 100 micron thick Velostat        1704 conductive film from 3M Corporation of St. Paul, Minn., and    -   III) a resilient separating layer 102 of 8 mil thick Duraflex        PT9300 polyurethane film from Deerfield Urethane of        Massachusetts. The 1^(st) conductive layer 101 was used as a        ground plane to shield the device from outside interference.

Next a 2^(nd) conductive layer 108 was made starting with the samenonwoven fabric as in protective layer 104. This was coated with aconductive paste consisting of a mixture of 60% Hycar 26-1199 binderfrom Noveon of Gastonia N.C., 10% SFG-15 graphite from Timcal of Bodio,Switzerland and 30% water. To make the paste, the graphite was added tothe water along with a approximately 10 mL of SL 6227 dispersant fromMilliken Chemical of Spartanburg, S.C., while stirring. Next the Hycarbinder was added. Finally, Acrysol RM-8W thickener from Rohm and Haas ofPhiladelphia, Pa. was added until the viscosity reached 12,000 cP asmeasured on a Brookfield viscometer.

This paste was screen printed onto the polyester nonwoven fabric tocreate the patterned structure shown in FIG. 6. Detector areas 120,traces 122, pin connections 124, and reference layer connections 126were printed. After printing, the fabric was dried in a forced-air ovenfor 15 minutes, to drive off the water and bind the coating to thefabric. Next, the conductive coating was painted with PE-001 silverpaste from Acheson Colloids of Port Huron, Mich. and placed back in theoven to dry. The female half of a penetrating pin connector (not shown)was attached, such that separate pins pierced the traces from the eachof the detectors.

This printed sheet was placed between the two identical resilientstructures, such that the nonwoven protective layers 104 were on theoutside of the resulting structure. Separate insulated copper wires wereattached to each conductive film to connect them to the groundconnections in the print. Adjacent layers were adhered together usingSuper 77 spray adhesive from the 3M Corporation of St. Paul, Minn.

The pin connector was attached to its male counterpart, which in turnwas attached to a shielded coaxial cable such that the ground sheath ofthe cable was connected to the 1^(st) conductive layers. The centerconductor was connected sequentially to each of the traces. The otherend of the coaxial cable was inserted into the capacitance measurementslots of a Triplett 2102 multimeter, which was set to measure smallcapacitance. The capacitance between each trace and the reference layerswas measured twice, first in the quiescent state, and then while pushingon the detector so as to maximally compress the resilient separatinglayer.

Example 2

A second structure was made, identical to that in Example 1 except thatthe polymer in the resilient separating layer 102 was replaced with 3mil thick M823 silicone film from Specialty Silicone Products ofBallston Spa, N.Y.

Example 3

A second structure was made, identical to that in Example 1 except thatthe polymer in the resilient separating layer 102 was replaced with 44mil thick T-1505 HypurCEL polyurethane foam from Rubberlite Incorporatedof Huntington, W.Va.

Table A shows the measured capacitances of each of the samples inExamples 1-3. These values are after subtracting out the 44 pF ofcapacitance between the cables leading from the panels to themultimeter. The pattern of the printed detector and trace elements isshown in FIG. 6. Detectors 120 were connected with the edge of thedevice buy the long trace 122 and the short trace 121. Also printed werethe ground connections 126 and the pin connections 124.

TABLE A Long trace (pF) 122 Short trace (pF) 121 Sample QuiescentPressed Quiescent Pressed Example 1 76 84 34 40 Example 2 70 83 41 56Example 3 29 39 14 24

Table A shows that a thinner dielectric will give a larger capacitance,but that a thicker dielectric may give a larger relative change incapacitance (relative to the quiescent value). Which is preferabledepends on the application environment, desired sensitivity, andresolution of the detecting electronics.

It is intended that the scope of the present invention include allmodifications that incorporate its principal design features, and thatthe scope and limitations of the present invention are to be determinedby the scope of the appended claims and their equivalents. It alsoshould be understood, therefore, that the inventive concepts hereindescribed are interchangeable and/or they can be used together in stillother permutations of the present invention, and that othermodifications and substitutions will be apparent to those skilled in theart from the foregoing description of the preferred embodiments withoutdeparting from the spirit or scope of the present invention.

1. A flexible capacitive sensor comprising: a first flexible, resilient dielectric layer having a first and second side, wherein the first flexible, resilient dielectric layer comprises a foam; an electrically conductive detector and trace layer on the first side of the first dielectric layer comprising at least 2 individually electrically addressed detectors and traces; a first electrically conductive reference layer on the second side of the first dielectric layer that completely overlaps the detectors; and, a capacitance meter electrically connected to each trace and the first conductive reference layer wherein the first conductive reference layer has a first voltage and the detector and trace layer has a second voltage, and wherein the first and second voltages have a difference of at least 0.1 volts.
 2. The capacitive sensor of claim 1, wherein the first dielectric layer compresses by 50% when a load of between 0.07 and 1.4 bar is applied.
 3. The capacitive sensor of claim 1, wherein the first dielectric layer compresses between 10 and 50% when a 0.14 bar load is applied.
 4. The capacitive sensor of claim 1, wherein the electrically conductive detector and trace layer comprises fabric printed with a conductive ink.
 5. The capacitive sensor of claim 1, further comprising an insulating layer on an outer surface of the capacitive sensor.
 6. The capacitive sensor of claim 5, wherein the insulating layer comprises graphics corresponding to the location of the detectors on the electrically conductive detector and trace layer.
 7. The capacitive sensor of claim 1, further comprising: a second flexible, resilient dielectric layer on the electrically conductive detector and trace layer on the side opposite the first flexible, resilient dielectric layer; a second electrically conductive reference layer on the second flexible, resilient dielectric layer on the side opposite the electrically conductive detector and trace layer; and, the capacitance meter additionally electrically connected to the second conductive reference layer.
 8. The capacitive sensor of claim 7, wherein the second reference layer has a third voltage, and wherein the second and third voltages have a difference of at least 0.1 volts.
 9. The capacitive sensor of claim 8, wherein the first voltage and the third voltages are equal. 